Ecohydrology Seminar Series: Dr. Lakshika Girihagama

Thursday, October 18, 2018 6:30 am - 7:30 am EDT

Join Dr. Lakshika Girihagama from the Department of Physical and Environmental Sciences at the University of Toronto Scarborough, as she presents "Physical circulation in the coastal zone of a large lake controls the benthic biological distribution".

There are gradients of conductivity and nutrients in the coastal zone of Eastern Georgian Bay of Lake Huron that appear to limit the spatial distribution of Dreissena mussels. Rivers flowing into Georgian Bay from the Canadian Shield are relatively low in conductivity compared to the main body of Lake Huron, and so there is an observed gradient of solute reminiscent near the river mouths. In order to understand the hydrodynamics of these freshwater estuaries (“freshtuaries”), we present modelling results using a conceptual one-dimensional model and a more complex three- dimensional hydrodynamic simulation (FVCOM) to show the mechanisms that may be responsible for calcium limitation in nearshore mussel distribution. We find that no mussels exist for specific conductivities less that 140 μS/cm. The observations show a strong positive correlation of conductivity and calcium concentration. Thus, we use conductivity as a proxy to infer the solute concentrations required for successful growth of Dreissena. Our freshestuarine models show the mixing length-scale of low conductivity waters from rivers in terms of geometry, horizontal dispersion, and the river influx. The resultant mixing length scale varies 2 – 6 km across the seasonally stratified and barotropic coastal waters in the Eastern Georgian Bay. The model predicted seasonally variable spatial position of this limiting solute concentration is in the range of 1 – 4 km from the freshtuary mouth. We will also, present results from tracer release numerical experiments in FVCOM to track transport and dispersion in river waters. Thus, our new finding shows a clear linkage between water circulation patterns setting nearshore chemical gradients, and hence species distribution gradients. This is significant because spread of Dreissena has impacts on nearshore ecology and on human infrastructure.